Fig. 17.28 ). As discussed previously, when a person stands, hydrostatic pressure increases in the legs and decreases in the head.
Tissue pressure, or, more specifically, interstitial fluid pressure (Pi) outside the capillaries, opposes capillary filtration. The difference between Pc and Pi constitutes the driving force for filtration. Normally, Pi is close to zero, and so Pc essentially represents the hydrostatic driving force.
Osmotic Forces. The key factor that restrains fluid loss from capillaries is the osmotic pressure of plasma proteins (such as albumin). This osmotic pressure is called colloid osmotic pressure or oncotic pressure (πp). The total osmotic pressure of plasma is approximately 6000 mm Hg (reflecting the presence of electrolytes and other small molecules, as well as plasma proteins), whereas oncotic pressure is only approximately 25 mm Hg. This low level of oncotic pressure is an important factor in fluid exchange across the capillary because plasma proteins are essentially confined to the intravascular space, whereas electrolytes are virtually equal in concentration on both sides of the capillary endothelium. The relative permeability of solute by water influences the actual magnitude of osmotic pressure. The reflection coefficient (σ) is the relative impediment to the passage of a substance through the capillary membrane. The reflection coefficient of water is 0, and that of albumin (to which the endothelium is essentially impermeable) is 1. Filterable solutes have reflection coefficients between 0 and 1. In addition, different tissues have different reflection coefficients for the same molecule. Hence, movement of a given solute across the endothelial wall varies with the tissue. The actual oncotic pressure of the plasma (πp) is defined by the following equation (see also
Equation 17.19 1 µm 0.5 µm 0.1 µm0.1 µm GJTJ CT Nu PC V BM * V A B C D •Fig. 17.26 A, Electronmicrographofacross-sectionofacapillaryinamouseventricle.Theluminaldiameterisapproximately4µm.Inthissection,thecapillarywallisformedbyasingleendothelialcell(Nu,endothelialnucleus).Thethinpericapillaryspaceisoccupiedbyapericyte(PC)andaconnectivetissue(CT)cell(“fibroblast”),whichformsafunctionalcomplex(arrow)withitself.V,plasmalemmalvesicles.B, DetailoftheendothelialcellinA showingplasmalemmalvesicles(V)attachedtotheendothelialcellsurface.Thesevesiclesareespeciallyprominentinvascularendotheliumandareinvolvedintransportofsubstancesacrossthebloodvesselwall.Notethecomplexalveolarvesicle(asterisk). BM,basementmembrane.C, Junctionalcomplexinacapillaryofamouseheart.“Tight”junctions(TJ)typicallyforminthesesmallbloodvesselsandappeartoconsistoffusionsbetweenapposedendothelialcellsurfacemembranes.D, Interendothelialjunctioninamusculararteryofapapillarymuscle.Althoughtightjunctionssimilartothoseofcapillariesarefoundintheselargebloodvessels,extensivejunctionsthatresemblegapjunctionsintheintercalateddisksbetweenmyocardialcellsoftenappearinarterialendothelium(exampleshownatGJ).
CHAPTER 17  Properties of the Vasculature 365
Junction of two Discontinuousendothelial cells endothelium • Fig. 17.27 Illustration of an electron micrograph of a capillary in cross-section.